1. Field of the Invention
The present invention relates to a method and means for compensating for the photomultiplier tube dark signal in a spectrophotometer and, more particularly, to a method and means for simultaneously compensating for the dark current of a photomultiplier tube and the offset of an operational amplifier in a spectrophotometer.
2. Description of the Prior Art
In a spectrophotometer, a beam of light of a known frequency is transmitted through a sample and a photomultiplier tube is positioned to detect the light passing through the sample. The less light absorbed by the sample, the more light is transmitted, and the output of the photomultiplier tube is a current signal proportional to the intensity of the detected light. Thus, the output of the photomultiplier tube is proportional to transmittance.
An operational amplifier is typically used to convert this current signal into a DC voltage signal. Such an operational amplifier typically has inverting and non-inverting inputs and an output whereupon the current from the photomultiplier tube is applied to the inverting input of the operational amplifier.
Photomultiplier tubes respond virtually instantaneously to light level changes and while this characteristic is generally desirable, it is undesirable in the sense that noise signals also affect the tube current and these signals are transmitted to the operational amplifier. In order to filter this noise, a low pass filter is generally coupled between the inverting input and the output of the operational amplifier so that the output of the operational amplifier responds slowly to current changes from the photomultiplier tube. A typical filter network might introduce a delay such that it takes approximately one-half second for the output of the operational amplifier to stabilize.
The use of an operational amplifier and a low pass filter for converting the current of a photomultiplier tube to a DC voltage creates a problem when compensating for the dark current of the photomultiplier tube. That is, under dark conditions, when there should be no light falling on the photomultiplier tube, there is still a current flowing therethrough, referred to as the dark current, and this current produces a proportional voltage at the output of the operational amplifier. An offset must be subtracted from this voltage to obtain a zero output voltage under dark conditions. In the past, various schemes have been utilized to compensate for the dark current of the photomultiplier tube so as to reduce the output voltage of the operational amplifier to zero volts under dark conditions. Such compensation techniques generally must be iterative, i.e. a compensation signal is applied to the circuit, the output voltage of the operational amplifier is sensed to determined whether it has been reduced to zero, and, if it has not, the compensation signal is adjusted and the output voltage of the operational amplifier again sensed. This procedure repeats until the output voltage of the operational amplifier is reduced to zero.
All such prior compensation techniques have left the filter network within the compensation loop so that the delay between successive iterations must be at least the one-half second it takes for the output of the amplifier to stabilize. When using a microcomputer with its rapid calculating capabilities to analyze the output of a spectrophotometer photomultiplier tube, such delay is undesirable in that compensation often requires several seconds to achieve.